1. Field of the Invention
The present invention relates to a process to transmit data between a read/write device and a transponder. The read/write device sends data to the transponder through induction.
2. The Prior Art
Systems consisting of read/write devices and transponders are for identifying objects, persons and animals without contact. The transponder is a data memory whose information can be read out and changed remotely with the read/write device. Passive transponder systems have become particularly wide-spread where the transponder is supplied with energy from the read/write device, and the transponder does not contain an energy source.
Energy and information are transmitted inductively. To increase the effectiveness of the transponder and its read/write device, oscillating circuits are used by both devices whose coils establish the coupling. The data is transmitted from the read/write device to the transponder by changing the field strength and evaluating the time between sequential field strength changes which are identified as pulse times.
These systems are frequently used at long range. This long range can only be obtained when the read/write device has a strong field. If the transponder is placed directly in front of the read/write device, it receives too much energy, and a very high voltage can arise at its oscillating circuit. To prevent the transponder from being destroyed when the oscillating circuit voltage is too high, the amplitude of the field is limited in the transponder.
The detected pulse times depend on the coupling between the oscillating circuits of the read/write device and the transponder, the parameters of the oscillating circuits, and the transmission strength of the read/write device. A fixed compensation for the changes or distortions is not possible, either in the read/write device or the transponder.
It is important to ensure that the potential distortion does not alter the information in the transmitted data. To measure this distortion, a first set of set points are created by an initial transmission of the signal of a set duration from the read/write device to the transponder. Therefore, the target difference between the two states' pulse times must be greater than the maximum possible time deviation of the times at which the field strengths change from the setpoints when transmitting two logical states. This increases the pulse times and consequently reduces the possible data rates that otherwise would only be determined by the smallest recognizable difference between the different pulse lengths.